1. Field of the Invention
This invention relates to determining the endpoint of development of photoresist, electron beam resist, X-ray resist and the like. More particularly, this invention relates to a system for controlling the end of development of a resist when a test pattern disappears.
2. Prior Art
Price, "Etch End-Point Detector," IBM Technical Disclosure Bulletin 15, 3532-3533 (1973) shows a laser source directed onto a metal or oxide film being etched from the surface of a wafer. Monochromatic laser light is transmitted to the wafer through fibers in light pipes to the wafer. Light reflected from the wafer is fed through other fibers in light pipes to a detector. Because of interference phenomena, the reflected signal varies in a sinusoidal manner as the film thickness changes during etching. A detector filters and differentiates that signal to produce an output which goes to zero as etching is completed.
U.S. Pat. No. 3,874,959 of Hoekstra et al entitled, "Method to Establish the Endpoint During Delineation of Oxides on Semiconductor Surfaces and Apparatus Therefor," describes a technique for detecting the endpoint of etching away a dielectric by sensing the amount of current passing through a substrate on which the dielectric is deposited in response to a pulsating light beam. The dielectric is in the form of an oxide which is highly resistant to electrical current, but which is etched away to permit current flow.
U.S. Pat. No. 3,669,673 of Ih et al entitled, "Recording of a Continuous Tone Focused Image on a Diffraction Grating," describes forming an image with a diffraction grating coated with photoresist. The purpose, however, was simply to record a holographic image in the grating pattern for displaying a hologram later. It has nothing to do with endpoint detection.
U.S. Pat. No. 3,622,319 of Sharp for "Nonreflecting Photomasks and Methods of Making Same" shows an array of opaque lines of a mask and optical patterns generated by reflection of light. This deals with a method of making a mask and has nothing to do with endpoint detection.
U.S. Pat. No. 3,728,117 of Heidenhain et al entitled, "Optical Diffraction Grid," shows production of diffraction gratings photographically, and has nothing to do with endpoint detection.
U.S. Pat. No. 3,708,229 of Pircher for a "System for Measuring Optical Path Length Across Layers of Small Thickness" describes an interferometery technique for measuring the thickness of a layer. It monitors the thickness of a thin film.
Automatic endpoint detection has been performed by means of projecting a beam of light at a thin film pattern at a predetermined angle to a thin film and placing a detector in line with the reflected light to monitor standing waves of diminishing amplitude caused by the reflected waves from the photoresist-air and photoresist-substrate interfaces.
Recent advances in lithography systems require minimum feature dimensions in the submicron region. Conventional development inspection with optical microscopes reveals large defects such as severely distorted images due to sizable mask-to-wafer gap, distinctly overexposed or underexposed areas due to significant variation of illumination intensity, and large micron-size defects. However, it is impossible to observe precisely the change of the submicron linewidths or to determine the complete removal of residual resist material in the exposed areas when they are on the order of 1/4 micron wide with 1000 Angstrom spaces between T and I bars. In such a case, it is impossible to see when development is completed. Because of long turnaround time and the unavoidable destructive exposure of a sample to radiation, the high resolution capability of scanning electron microscopy (SEM) cannot be utilized except for the purpose of monitoring occasional samples in a manufacturing system. It is preferred to test for development on each and every wafer.
An object of this invention is to enable submicron linewidth control with a low power microscope. It is applicable for electron-beam, X-ray, or photographic lithography as long as a positive resist is used.
A further object of this invention is to ensure complete removal of photoresist residue during development.
Another object of this invention is in situ inspection of the state of development of a sample during a process of photoresist development.
In accordance with this invention a manufacturing system is provided for controlling development of a resist pattern on a film of resist deposited on a substrate including a film of resist exposed to a pattern in a first area of its surface and exposed in a monitoring area of its surface to a pattern with a different degree of exposure and means for optically monitoring the monitoring area during development of the resist to determine when the resist has been substantially removed from the monitoring area to provide a signal indicating a shift in light intensity reaching a predetermined location for the purpose of providing an indication of the endpoint of development of the pattern upon the first portion.
Further, in accordance with this invention, a small test area on a wafer is blanket exposed independently from the exposure of a pattern projected onto the entire wafer. Preferably, the amount of blanket exposure in the small area is held to a fixed relationship with a low power microscope. When the patterns in the test area disappear completely, the desired level of development has been accomplished.